Most of the transmission paths between central offices in larger cities in this country is by way of short-haul multipair cable. Such multipair cable trunks typically consist of 24-gauge pulp-insulated cable for voice frequency paths, and 22-gauge pulp-insulated conductors for carrier circuits. The cables typically are placed underground in ducts. Pulp-insulated conductor cable has been the industry's standard for such short-haul routes in the past because, inter alia, of its close packing providing an optimally large number of conductor pairs per unit cross section of cable.
Once installed, these short-haul cables at first carry mainly voice frequency signals. As the trunk traffic grows, and each pair is placed into interoffice use, it is economical to defer duct construction and cable placement by installing on the existing cable pairs multiplexing systems known as carrier. The pattern of utilizing a cable first for voice frequency circuit pairs and later--as more channels are needed--adding carrier circuits to the pairs, has proven highly cost-efficient for some situations. This efficiency was and is particularly true in the case of T1 carrier, a now-widely used multiplexing system which is described in The Bell System Technical Journal, Vol. XLIV, No. 7. This article to the extent relevant is hereby incorporated by reference.
The T1 carrier system was specifically designed to operate with standard 83 nanofarads per mile, 22-gauge wood pulp insulated conductor exchange area cable with a nominal repeater spacing of 6,000 feet and a repeater gain of 35 dB. When installed, the system converts each two pairs to 24 voice-frequency channels. However, with the steady growth of interoffice short-haul trunks there is still further incentive to improve upon the design of short-haul transmission systems.
One significant further factor in the design of a modern cost-efficient interoffice short-haul plant is that many offices are connected by a composite of voice frequency signal paths and carrier paths. Existing multipair cable designs are not well adapted, however, to carry such composite traffic.
With the above and other factors in mind, the following are objects of the present invention:
TO ASSURE AN INTEROFFICE TRUNK TRANSMISSION SYSTEM WITH A WIDE FLEXIBILITY TO HANDLE BOTH VOICE AND CARRIER FREQUENCY SIGNALS;
TO PROVIDE SUCH A SYSTEM IN WHICH THE CABLE CONFIGURATION PERMITS THE SAME T1 repeater spacing and gain as the present standard;
to minimize copper usage;
importantly, to maximize pair counter per unit cable cross section for metropolitan conduit utilization efficiency surpassing that obtainable with 22-gauge wood pulp;
to provide such an interoffice transmission system in which the cable component electrically approximates the voice frequency transmission characteristics of 24-gauge pulp cable with or without the present conventional H88 loading, thus to facilitate utilization of existing facilities;
to minimize voice frequency equipment redesign;
for a given pair count in a cable, to achieve a cable design that further minimizes incidence of crosstalk, thus to permit economic use thereon of carrier frequencies yet higher than T1; and
overall, to enhance the efficient growth pattern whereby cable pairs may be utilized first for voice circuits and then, as growth demands, converted to T1 carrier transmission.